The invention relates to a primary X-ray diaphragm for an X-ray examination apparatus, with a plurality of diaphragm plates which define the cone of rays, the diaphragm plates being adjustable in a direction perpendicular to the symmetry axis of the primary X-ray diaphragm and being rotatable about the symmetry axis.
A primary X-ray diaphragm of the type referred in the preceding paragraph is already obvious from U.S. Letters Patent No. 3,287,561. With such a primary X-ray diaphragm, a rectangular or a square cone of rays can be delineated. Depending on need, the diaphragm can be rotated about the symmetry axis which most often coincides with the central ray from the X-ray tube to correspondingly rotate the cross section of the transmitted cone of radiation. A primary X-ray diaphragm with a rectangular diaphragm aperture which can be rotated about the symmetry axis meets the requirements of being able to completely utilize a cassette format or a film format, and to selectively adapt to either format. However, particularly in the examination of organs which border directly on regions of the body having a strongly reduced ray absorption, a disadvantage arises with respect to this primary X-ray diaphragm as a consequence of the rounded boundary contours of the organ which are to be recorded near the corners of a rectangular image format; in particular the radiation which passes laterally of the boundary contours of the organ to be examined and strikes the film, is only slightly attentuated as it passes through the adjoining body regions and thus over exposes the areas of the film corresponding to the boundary regions of the organ under examination. The consequence of this is that the recognizability of details in the areas of the film corresponding to such boundary regions is impaired. A typical example of this is in heart examinations where the coronary vessels are obscured as a result of the radiation which passes laterally adjacent to the heart and which is only slightly attenuated by the lung tissue.
From U.S. Letters Patent No. 3,631,249, it is obvious to delineate a circular diaphragm aperture in the case of primary X-ray diaphragms. These diaphragms serve the purpose of adjusting the cone of rays in conformity to the likewise-circular entry window of an X-ray image intensifier. These diaphragms of themselves do not admit of any other aperture cross section.